1. Field of the Invention
This invention relates in general to ceramic electrolytes and fuel cell devices utilizing them, and to the laser micromachining of electrolyte sheets and electrolyte supported multi-cell solid oxide fuel cell devices.
2. Technical Background
The present invention pertains to articles formed by laser processing of solid oxide fuel cell electrolyte sheets, as well as manufacture of electrolyte supported solid oxide fuel cells and fuel cell devices.
Solid oxide fuel cell devices incorporating flexible ceramic electrolyte sheets are known. In such fuel cell devices, often one or more electrolyte sheets are supported within a housing, on a frame, or between a pair of mounting assemblies, which might be either a frame or a manifold. The electrolyte sheets may be utilized either in a multi-cell or single cell design.
A common approach utilizes a fuel cell device that consists of a single cell design where the thickest component of the fuel cell is an anode layer. This anode layer acts as both support and catalyst and can be about 100 to 1000 microns in thickness and is often formed from a composite of nickel and yttria stabilized zirconia. Such single cells further include a thin electrolyte layer overlying the anode layer, and a cathode layer overlying the electrolyte.
In a multi-cell design, such as that disclosed in U.S. Pat. No. 6,623,881 assigned to Corning Incorporated, the fuel cell device includes an electrolyte sheet in the form of a thin ceramic sheet (e.g., zirconia doped with yttrium oxide (Y2O3)). The zirconia based electrolyte sheet may be 20-30 microns thick. Typically, the doped zirconia electrolyte sheet supports a plurality of cells, each of which is formed by an anode and cathode layer on either side of the doped zirconia sheet. The thin pre-sintered electrolyte sheet can support either a single anode and cathode pair, thereby forming a one cell device, or multiple anodes and cathodes and a plurality of cells are fabricated on a common electrolyte substrate and are interconnected, through the thickness of the electrolyte sheet by the conductive via connectors (vias).
In order to avoid fracturing of electrolyte sheets, the fuel cell device fabrication process typically utilizes mechanical punching of the via holes and mechanical cutting of the device edges while the electrolyte sheet is in the un-fired state. The process of mechanically punching of unfired ceramic electrolyte sheets requires predicting the sintering shrinkage of a particular electrolyte batch in particular furnace conditions. If the prediction is off, the punched via holes will be misaligned after sintering. After punching and cutting, the electrolyte is fired and typically undergoes 15% to 30% linear shrinkage due to the de-binding and sintering process. Larger electrolyte pieces require better accuracy in shrinkage values to maintain the tolerances needed for device fabrication, especially with multi-cell devices. For example, an electrolyte length of 50 cm and via hole positioning tolerances of +/−200 μm in the sintered state, corresponds to predicting the electrolyte shrinkage by better than +/−0.05%. Mechanically punching and cutting of the un-fired electrolyte puts limitations on the fabrication speed, feature size, wrinkle, and edge quality produced. Also, machining of parts in the un-fired state requires an accurate prediction of part shrinkage in order to maintain dimensional tolerances. Such prediction is very difficult to do with the desired accuracy and require actual devices to be sacrificed for testing.
The general use of laser micromachining thick ceramics is known. It is applicable to machining of bulk ceramic pieces with thickness of 250 μm or larger, and not thin electrolyte films of thickness of less than 50 μm. Thin (less than 50 μm) zirconia based sintered electrolyte sheets are brittle when they are either cut or/and drilled by mechanical means, due to crack formation.
The process of forming via holes in sintered ceramic substrates for electronic components is described in U.S. Pat. No. 6,270,601. This patent discloses use of either mechanical or laser drilling of thick sintered ceramic substrate with thicknesses of 3-60 mils (76.2 to 1524 μm). This reference suggests that laser drilling of sintered ceramic pieces may be achieved by using either CO2 or excimer laser systems. No details are provided on how to laser machine via holes in sintered electrolyte sheets. Applicants attempted to utilize CO2 laser in drilling thin zirconia ceramic electrolyte sheets, but were not successful due to a large number of cracks created by thermal effects. U.S. Pat. No. 6,270,601 also provided no guidance on how to utilize excimer laser for successful cutting or drilling of the electrolyte sheets.
US patent publication No 2002/0012825 describes a fuel cell electrolyte sheet with 3-dimensional features micromachined on its surface. This application does not teach or suggest that it is possible to laser machine electrolyte sheets after sintering.
Prior efforts to produce flat electrolyte of thicknesses greater than 50 μm has led to waves or dimples and edge burrs as described in European Patent EP 1063212B1. This reference discloses stacking of electrolyte sheets during sintering, to limit the wave and burr heights to under 100 μm. The reference teaches that zirconia sheets and other ceramics sheets are brittle when subjected to external forces in a bending direction. In contrast, fuel cells formed from thin flexible electrolyte can withstand significant bending without failure. However, they also develop edge curl when sintered, and the edge curl can produce stress, and fracture the sheet when the curl is flattened.